1. Field of the Invention
The present invention relates to a voltage regulator that outputs a constant voltage, and more particularly to an overcurrent protective circuit that reduces an output current to protect a circuit when an overcurrent flows into an output terminal.
2. Description of the Related Art
Voltage regulators have been employed as voltage supply sources of circuits in diverse electronic devices. The function of the voltage regulator is to output a constant voltage to the output terminal without being affected by a voltage variation of an input terminal. Also, it is important that the voltage regulator functions as overcurrent protection that reduces an output current to protect a circuit when a current that is supplied to a load from the output terminal increases and exceeds a largest current (for example, refer to JP 2003-29856 A).
FIG. 5 shows a circuit diagram showing a voltage regulator having an overcurrent protective circuit. The conventional voltage regulator having the overcurrent protective circuit includes an output voltage divider circuit 2 that divides a voltage at an output terminal VOUT, a reference voltage circuit 3 that outputs a reference voltage, an error amplifier 4 that compares the divided voltage with the reference voltage, an output transistor 1 that is controlled by an output voltage of the error amplifier 4, and an overcurrent protective circuit 100. The overcurrent protective circuit 100 includes an output current detection transistor 5 and a detection resistor 6 which are an output current detector circuit that is connected in parallel to the output transistor 1, and a transistor 7, a resistor 8, and an output current control transistor 9 which constitute an output current limiter circuit that is controlled by a voltage of the detection resistor 6.
The above overcurrent protective circuit 100 has a function of protecting a circuit from the overcurrent with the following operation.
In the case where the output current of the output terminal VOUT increases, the detection current that is in proportion to the output current flows in the output current detection transistor 5. The detection current flows in the resistor 6, thereby allowing a voltage between the gate and the source of the transistor 7 to rise. In this case, when the overcurrent flows in the output terminal VOUT, and the voltage between the gate and the source of the transistor 7 exceeds a threshold voltage due to the detection current that is proportional to the overcurrent, a drain current flows in the transistor 7. Accordingly, the voltage between the gate and the source of the output current control transistor 9 drops, and a drain current flows in the output current control transistor 9, thereby allowing the voltage between the gate and the source of the output transistor 1 to rise. With the execution of feedback as described above, the gate of the output transistor 1 is so controlled as to hold the drain current of the output current detection transistor 5 constant. As a result, an increase in the output current is suppressed.
However, the output current detection transistor 5 of the overcurrent protective circuit 100 suffers from such a problem that because the drain voltage changes according to the input voltage, a relationship of current between the output current detection transistor 5 and the output transistor 1 is collapsed due to the channel length modulation effect, to thereby deteriorate a precision in the detection of the overcurrent.
Accordingly, the overcurrent protective circuit 100 needs to make a voltage VA at the drain (point A) of the output current detection transistor 5 identical with a voltage VB at the drain (point B) of the output transistor 1, and uses a current mirror circuit as a circuit for achieving the above requirement.
The operation will be described below. A current of the same amount as that of the detection current flows by the transistor 11 that is identical in size with the output current detection transistor 5. The current is reflexed by a first current mirror circuit, and flows in transistors 14, 15, and 16 that constitute a second current mirror circuit, thereby making the voltage VA at the point A identical with the voltage VB at the point B.
However, the circuit using the above current mirror circuit has a drawback that a current consumption increases because the same current as that of the detection current flows in two paths that pass through transistors 11, 15, and 12 and transistors 14 and 13, respectively.
The present invention has been made to solve the above problems, and an object of the present invention is to provide an overcurrent protective circuit that is high in detection precision without increasing the current consumption.